Another Transmition question (1997)

I have a 1997 explorer sport with 90,000 miles. I have been having problems lately with it when it shift in to 4th or 5th gear. It shift fine untill you get around 40-50 mph When it get to this point it runs around 2000 rpms and it waits about 5 seconds to shift and it finally does then it drops the rpms to around 1500. It not a hard shift when it does it. I had the trans fluid changed about two months ago dealer didn't say anything about any smell from the fluid or metal shavings. I have heard about trans going out and about a electronic device that is under the passanger side of car that can go bad. Just wondering if anyone else has had the same problem as me and if it was the trans or the electronic device. Thanks